In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is at least partially evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, the print quality is inconsistent. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is at least partially dried on the blanket and then transferred to media. Such a printer avoids the changes in image quality, drop spread, and media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is jetted on to an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality the ink drops jetted onto the blanket must spread and well-coalesced prior to drying, otherwise, the ink images appear grainy and have deletions. The lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface. In order to facilitate transfer of the ink image from the blanket to the media substrate, however, a blanket having a surface with a relatively low surface energy is preferred. These diametrically opposed and competing properties for a blanket surface make selections of materials for blankets difficult. Reducing ink drop surface tension helps, but the spread is still generally inadequate for appropriate image quality.
One problem confronting indirect aqueous inkjet printing processes relates to the spread of ink drops during the printing process. Indirect image receiving members are formed from low surface energy materials that promote the transfer of ink from the surface of the indirect image receiving member to the print medium that receives the final printed image. Low surface energy materials, however, also tend to promote the “beading” of individual ink drops on the image receiving surface. The resulting printed image may appear to be grainy and solid lines or solid printed regions are reproduced as a series of dots instead of continuous features in the final printed image.
An optimum blanket for an indirect image transfer process must tackle three challenges: 1) wet image quality; 2) image transfer; and 3) print-head management. The first challenge—wet image quality—prefers a high surface energy density which causes the aqueous ink to spread and wet the surface, rather than beading up into discrete droplets. The second challenge—image transfer—prefers that the ink, once partially dried, has minimal attraction to the blanket surface so that 100% of the ink is transferred to the media substrate. Thus, image transfer is optimized by minimizing surface energy. The third challenge relates to how well the print head carrying the ink jets can be kept clean of dried ink. For resin-based ink, the drying of the ink on the face plate of a print head can render it inoperable. On the other hand, too much moisture can condense on the face plate and cause jetting problems. In addition, some ink jets can be sensitive to high temperatures, typically temperatures above about 70° C.
Various approaches have been investigated to provide a solution that balances all three challenges, including blanket material selection, ink design and auxiliary fluid methods. With respect to material selection, materials that are known to provide optimum release properties include the classes of silicone, fluorosilicone, TEFLON, VITON and certain hybrid materials. These compositions have low surface energy but provide poor in wetting. Alternatively, polyurethane, and polymide have been used to improve wetting but at the cost of poor ink release properties. Tuning ink compositions to address these challenges has proven to be very difficult since the primary performance attribute of the ink is the performance in the print head. For instance, if the ink surface tension is too high it will not jet properly and it if is too low it will drool out of the face plate of the print head. Compounding the problem is the fact that ink cohesion must be significantly greater than the ink-to-blanket adhesion for all image contents, including the stress cases of single layer small dot and three layer process black solid printing
Thus far, the balance between the three challenges has been elusive. Most solutions have tended to err toward optimizing image transfer from the blanket to the media substrate, with some sacrifice to image quality. What is needed is a low-cost solution to this problem that optimizes both wet image quality and image transfer without compromising the ink jet print head.